1. Field of the Invention
This invention generally relates to an apparatus and a method for producing images of the interior of a body, such as a human body, using data derived from NMR measurement techniques, and more particularly to an improved apparatus and method for producing synthesized computed images based on a minimal number of NMR measurements.
2. Description of the Prior Art
The phenomenon of nuclear magnetic resonance (NMR) has been utilized in recent years to produce images of the interior portions of the body, particularly human bodies, for diagnostic and other purposes. Although primarily used for examination of the interior of a body or body portion having a variety of relatively soft tissues, NMR techniques can also be utilized under other widely varying circumstances. NMR images are utilized to portray characteristics of tissues under inspection which cannot be readily obtained using radiographic techniques and other imaging methods.
A fundamental description of basic NMR physics and imaging techniques is found in Kaufmann et al, Nuclear Magnetic Resonance Imaging in Medicine, Igaku-Shoin Medical Publishers, Incorporated, New York and Tokyo (1981). The important features of the NMR technique which are commonly used in previous methods will now be elaborated upon.
The body under examination is subjected to one magnetic field which is usually constant in magnitude and another field which lies along at least one different vector from the first magnetic field. This second magnetic field is usually time-varying with the exact characteristics of each of these fields being a function of choice between any one of several available imaging techniques. During NMR examinations, the characteristics of the field energy must be preselected. For example in one technique called spin-echo imaging, the repetition time T.sub.R, which is defined as the time between successive applications of the same pulse sequence, and the sampling delay time .tau. (TAU), must be pre-selected. The first and second fields are applied to the tissue being examined in accordance with selected times and the results are detected and stored with the levels of the storage signals being correlated with their physical positions and with these levels being represented by a matrix of numbers. The numbers of the matrix are then displayed as a matrix of points or pixels which have different light or dark levels and the composite of these pixels forms an image having various contrasting areas.
A physician utilizes the results of these contrasting areas within the image to observe and analyze a "slice" of the body of which the image was made from and, in a medical context, to thereby form a diagnosis. The degree of contrast between the various areas is a function of T.sub.R and .tau. values which are selected before the measurement is made and also the intrinsic properties of the materials including net magnetization M.sub.o (which is proportional to Proton density), and the relaxation times T.sub.1 and T.sub.2. It must be noted that one particular set of values for T.sub.R and .tau., will only produce an image with excellent contrast between certain sets of materials but this contrast will be insufficient between other materials being used. The result of this varying contrast depending upon the materials provides for the necessity of making numerous sets of measurements with various values for T.sub.R and .tau.. The images which result from these measurements using these different values of T.sub.R and .tau. are used in order to adequately examine various tissues involved.
This is a time consuming process in which the patient is often subjected to discomfort and, additionally, is repeatedly subjected to the effects of a strong, unit directional magnetic field. For certain measurements, body movement must be prevented during each measurement and the duration of each measurement may last in the order of seconds or minutes, depending upon the pulse sequence technique being utilized and the materials under observation. These conditions establish rather severe limitations on the length of time which can be taken for such measurements and therefore, the number of measurements which can be made. Furthermore, it is not always possible for the examining physician to recognize in advance or even at that particular time, which materials are most significant in the images.
Additional background information and disclosure of devices and techniques in the field to which this invention relates can be found in the following articles and U.S. Patents.
Wehrli, F. W., J. R. MacFall, and G. H. Glover. The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and operator-selectable parameters. Presented at the meeting of the SPIE, Medicine XII, volume 419, April, 1983.
Young, I. R. et al. Contrast in NMR Imaging. Presented at the Society of Magnetic Resonance in Medicine, August 1983.
Ortendahl D. et al Calculated NMR images. Presented at the Society of Magnetic Resonance in Medicine, August 1983.
Davis, P. L., et al. Optimal spin-echo images for liver lesions by retrospective calculations. Presented at the Society of Magnetic Resonance in Medicine, August 1983.
______________________________________ U.S. PAT. NO. INVENTOR ______________________________________ 3,789,832 Damadian 4,045,723 Ernst 4,284,948 Young 4,292,977 Krause et al 4,297,637 Crooks et al 4,307,343 Likes 4,318,043 Crooks et al 4,354,499 Damadian 4,355,282 Young et al 4,390,840 Ganssen et al ______________________________________